Optical transmission has become the darling of communication technology because of the enormous bandwidth that is available on an optical fiber. Such bandwidth enables thousands of telephone conversations and hundreds of television channels to be transmitted simultaneously over a hair-thin fiber that is generally made from a high-quality glass material. Transmission capacity over an optical fiber is increased in WDM systems wherein several channels are multiplexed onto a single fiber--each channel operating at a different wavelength. However, in WDM systems, nonlinear interactions between channels, such as 4-photon mixing, severely reduces system capacity. This problem has been largely solved by U.S. Pat. No. 5,327,516 that discloses an optical fiber that reduces these nonlinear interactions by introducing a small amount of chromatic dispersion at the operating wavelengths. Accordingly, it is desirable for an optical fiber to provide a small amount of chromatic dispersion to each of the WDM channels. And while the presence of dispersion is desirable for the purpose of minimizing 4-photon mixing, it is undesirable because it causes pulse spreading due to the fact that different wavelengths travel at different speeds through the fiber. Fortunately, pulse spreading can be dealt with by a dispersion-compensation technique wherein alternating sections of positive and negative dispersion fiber are concatenated. Normally, dispersion compensation is not required for communication systems that are shorter than about 50 kilometers.
Important advances have been made in the quality of the glass material (nearly pure silica--SiO.sub.2) used in making optical fibers. In 1970, an acceptable loss for glass fiber was in the range of 20 dB/km; whereas today, losses are generally below 0.25 dB/km. Indeed, the theoretical minimum loss for glass fiber is about 0.16 dB/km, and it occurs at a wavelength of about 1550 nanometers (nm). Nature appears to favor optical transmission in this wavelength region because this is where Erbium-doped fiber amplifiers operate, and they have become the most practical optical amplifiers available. In such an amplifier, the Erbium ions within an optical fiber are "pumped" with energy in a first wavelength region (e.g., 980 nm), and then release that energy into a second wavelength region (e.g., 1530-1565 nm) when the Erbium ions are stimulated by optical signals in that second wavelength region.
Numerous considerations are involved in the design of an optical fiber that must necessarily cooperate to provide a commercially acceptable product. In general it is desirable for transmission loss to be low; for the fiber to be able to tolerate a modest amount of bending without experiencing excessive loss; for the fiber to have a known dispersion over a predetermined wavelength range; for the dispersion slope to be relatively low, and for the fiber to have a cutoff wavelength that is appropriate for singlemode transmission at the system wavelength. As discussed, high quality glass materials have been developed that provide low transmission loss, but high quality glass is insufficient in itself to satisfy all of the desired features of modern optical fibers. Many desirable features need to be addressed by the refractive-index profile of the fiber, which describes how its index of refraction varies as a function of distance from its central axis. Parameters used for describing the refractive-index profile are generally referenced to the index of refraction of the outermost layer of glass. Idealized models of refractive-index profile frequently comprise axially symmetric rings of different refractive index. However, changing the size and shape of any one of these rings generally impacts more than one characteristic of the fiber (e.g., dispersion slope may be reduced, but transmission loss is increased); and it is a significant design effort to create a refractive-index profile that provides all of the desired features and is still easy to manufacture.
For example, U.S. Pat. No. 5,878,182 discloses designs for positive and negative dispersion fibers that have a low slope in the Erbium amplifier region. And while these designs are effective to achieve a desired result, the manufacturing tolerances of the negative-dispersion fiber shown in FIG. 3C of this patent are tighter than desirable.
Another optical fiber that provides a low-dispersion slope across the Erbium amplifier region has a refractive-index profile that resembles a donut, and it is shown at pages 259-260 of the OFC '95 Technical Digest in an article entitled: Dispersion-shifted single-mode fiber for high-bit-rate and multiwavelength systems. This design comprises a ring of high index material surrounding a core of low index material. However, such an index profile would appear to have higher transmission loss and/or higher bending sensitivity than is desirable.
Accordingly, what is desired, but does not appear to be disclosed in the prior art, is an easily manufactured optical fiber that exhibits low transmission loss, low bending sensitivity, and negative dispersion with a low slope in the Erbium amplifier region.